This invention relates to an improvement in the off-gas desulfurization process which comprises fixing sulfur dioxide contained in the off-gas in the form of gypsum for removal therefrom by using an absorbing solution containing an organic carboxylate.
Attention has recently been focused, because of its advantages in desulfurization efficiency and economy, on a wet off-gas desulfurization process which comprises contacting sulfur dioxide contained in an off-gas with an absorbing solution containing an organic carboxylic acid and a salt thereof, and oxidizing the resulting absorbed solution while neutralizing the solution with limestone in order to fix the sulfur dioxide in the form of gypsum for separation.
In this process, the combination of an absorption reaction of sulfur dioxide with an absorbing solution containing an organic carboxylate and a neutralization reaction of the absorbed solution with limestone improves the desulfurization effect and remarkably promotes the gypsum-forming reaction, resulting in a reduction in both plant cost and power consumption.
The advantages mentioned above come about because, in this process, an absorbing solution containing an organic carboxylate is used to form a mixed system of carboxylate ion (RCOO.sup.-) and carboxylic acid (RCOOH) so as to cause the solution to have a good pH buffer capacity in a pH range of from 3 to 6, and because the carboxylate ion and the carboxylic acid may be present in a dissolved state in the absorbing solution containing a calcium compound such as limestone, gypsum, or the like.
This wet off-gas desulfurization process for removing sulfur dioxide in an off-gas by converting the sulfur dioxide to gypsum, usually called the gypsum by-producing process, essentially consists of an absorption reaction of sulfur dioxide with an absorbing solution, an oxidation reaction of the absorbed solution with an oxygen-containing gas such as air, and a neutralization reaction of the absorbed solution with limestone. Each reaction causes a pH change in the absorbing solution and, among the reactions, the higher the pH, the more effectively the absorption reaction of sulfur dioxide takes place, while a lower pH is preferable for the neutralization reaction with limestone. Therefore, in order to conduct each reaction as effectively as possible, it is necessary that the pH of the absorbing solution be maintained in a weak acidic range of from 3 to 6 and it is preferable that the pH change during each reaction be small.
In the off-gas desulfurization process described above, the use of a liquid containing an organic carboxylate as the absorbing solution causes each reaction to take place effectively, because the organic carboxylate has a good pH buffer capacity in a pH range of from 3 to 6.
However, a serious problem which results from the use of an absorbing solution containing an organic carboxylate is that the organic carboxylate is decomposed to be consumed during the oxidation reaction.
In other words, when SO.sub.3.sup.2- and HSO.sub.3.sup.- formed by the absorption of sulfur dioxide in the absorbing solution containing the organic carboxylate are oxidized with oxygen in the oxidation step for the absorbed solution, an active radical is formed as an intermediate reaction product, and the radical is responsible for the induced oxidative decomposition of the organic carboxylic acid coexisting therein.
The present inventor previously proposed, as a process for inhibiting the decomposition of organic carboxylic acid described above, a process in which a solution coexisted with a chloride ion in an amount of more than 2 percent by weight to a solution containing an organic carboxylate is used as an absorbing solution (see U.S. Pat. No. 4367205). The above process is based on the discovery that the decomposition of organic carboxylate is inhibited by the presence of a chloride ion, and that the higher the concentration of the chloride ion, the higher the inhibiting effect thereof.
However, although the above process is recommended for the desulfurization of an off-gas containing a large amount of hydrogen chloride such as coal combustion off-gas or in the event that the industrial water used contains a chloride ion at a high level, the above process is not recommended for the desulfurization of an off-gas containing chlorine compounds in small amounts such as a petroleum combustion off-gas, or in the event that industrial water containing a chloride ion at a low level is used because it is necessary to add a chlorine compound to the absorbing solution and provide an apparatus made of a corrosion resistant material in order to inhibit corrosion due to the effect of the chloride ion on the apparatus.
Moreover, further studies showed that, based on the above process, the dissolution rate of limestone during the neutralization step is reduced as the chloride ion concentration in the absorbing solution is increased, resulting in the need to increase the size of the neutralization tank in order to promote the neutralization reaction.